The present invention relates to a nitride based semiconductor device, and more particularly to a nitride based semiconductor photo-luminescent device.
A nitride based semiconductor is an extremely important material for a blue-color laser diode. In Jpn. J. Appl. Phys. Vol. 36 (1997), pp. L1568-1571, Nakamura et al. reported that a continuous photo-luminescence life-time over 10,000 hours at 2 mW output at room temperature was confirmed. FIG. 1 is a fragmentary cross sectional elevation view illustrative of a first conventional nitride based semiconductor laser diode. In order to increase the life-time of the laser diode, it is essential to reduce the dislocation density of the active layer. The first conventional nitride based semiconductor laser diode has the following structure. A stripe-shaped silicon dioxide mask 103 having window regions is selectively formed over a gallium nitride layer 102 over a top surface of a sapphire substrate 101, so that a gallium nitride layer is grown over the stripe-shaped silicon dioxide mask 103 and over the gallium nitride layer 102, wherein the gallium nitride layer has low dislocation density regions 104 which are positioned over the stripe-shaped silicon dioxide mask 103. A p-electrode 105 is positioned over the low dislocation density region 104.
A gallium nitride layer 102 is grown on the sapphire substrate 101 by a metal organic chemical vapor deposition method. The sapphire substrate 101 has a high surface dislocation density. The gallium nitride layer 102 has a high dislocation density. A stripe-shaped silicon dioxide mask 103 having window regions is formed over the gallium nitride layer 102 in a [1−1,0,0]-direction. A gallium nitride layer 106 is grown by a metal organic chemical vapor deposition method using the stripe-shaped silicon dioxide mask 103 with the window regions. The gallium nitride layer 106 has a high dislocation density region 116 and a low dislocation density region 104. The high dislocation density region 116 is formed on the gallium nitride layer 102 having a high dislocation density. The high dislocation density region 116 is grown in a vertical direction from gallium nitride layer 102 having a high dislocation density shown by the window regions of the stripe-shaped silicon dioxide mask 103, for which reason the dislocation is propagated from the gallium nitride layer 102 to the high dislocation density region 116, whereby the high dislocation density region 116 has a high dislocation density. The low dislocation density region 104 is formed over the stripe-shaped silicon dioxide mask 103. The low dislocation density region 104 is grown by the epitaxial lateral overgrowth from the window regions of the stripe-shaped silicon dioxide mask 103. The stripe-shaped silicon dioxide mask 103 cuts the further propagation of the dislocation from the sapphire substrate 101. The low dislocation density region 104 has a low dislocation density. At the center of the low dislocation density region 104, epitaxial lateral overgrowths of gallium nitride in various lateral directions from the window regions of the stripe-shaped silicon dioxide mask 103 come together, whereby new dislocations are formed, for which reason the center region of the low dislocation density region 104 has a high dislocation density. As a result, an Si-doped n-type gallium nitride epitaxial lateral overgrowth substrate 100 is completed which has the high dislocation density region 116 and the low dislocation density region 104.
An Si-doped n-type In0.1Ga0.9N layer 107 is formed over the Si-doped n-type gallium nitride epitaxial lateral overgrowth substrate 100. An n-type cladding layer 108 is formed over the Si-doped n-type In0.1Ga0.9N layer 107, wherein the n-type cladding layer 108 comprises 120 periods of alternating laminations of an Si-doped n-type GaN layer having a thickness of 2.5 nanometers and an undoped Al0.14Gao0.86N layer having a thickness of 2.5 nanometers. An Si-doped n-type GaN optical confinement layer 109 having a thickness of 0.1 micrometers is formed over the n-type cladding layer 108. A multiple quantum well active layer 210 is formed over the Si-doped n-type GaN optical confinement layer 109, wherein the multiple quantum well active layer 210 comprises alternating laminations of Si-doped n-type In0.15Ga0.85N quantum well layer having a thickness of 3.5 nanometers and an Si-doped n-type In0.2Ga0.98N potential barrier layer having a thickness of 10.5 nanometers. An Mg-doped p-type Al0.2Ga0.8N cap layer 111 having a thickness of 20 nanometers is formed over the multiple quantum well active layer 210. An Mg-doped p-type GaN optical confinement layer 112 having a thickness of 0.1 micrometer is formed over the Mg-doped p-type Al0.2Ga0.8N cap layer 111. A p-type cladding layer 113 is formed over the Mg-doped p-type GaN optical confinement layer 112, wherein the p-type cladding layer 113 comprises 120 periods of alternating laminations of an Mg-doped p-type GaN layer having a thickness of 2.5 nanometers and an undoped Al0.14Ga0.86N layer having a thickness of 2.5 nanometers. An Mg-doped p-type GaN contact layer 114 having a thickness of 0.05 micrometers is formed over the p-type cladding layer 113. The lamination structure over the Si-doped n-type gallium nitride epitaxial lateral overgrowth substrate 100 is selectively removed by a dry etching process to form a ridge structure over a predetermined region of the top surface of the Si-doped n-type gallium nitride epitaxial lateral overgrowth substrate 100, wherein the ridge structure comprises laminations of the Si-doped n-type In0.1Ga0.9N layer 107, the n-type cladding layer 108, the Si-doped n-type GaN optical confinement layer 109, the multiple quantum well active layer 210, the Mg-doped p-type Al0.2Ga0.8N cap layer 111, the Mg-doped p-type GaN optical confinement layer 112, the p-type cladding layer 113 and the Mg-doped p-type GaN contact layer 114. The Mg-doped p-type GaN contact layer 114 is positioned over the low dislocation density region 104 for current injection into the low dislocation density region 104. A p-electrode 105 is formed on the Mg-doped p-type GaN contact layer 114, wherein the p-electrode 105 comprises an Ni layer and an Au layer. An n-electrode 115 is also selectively formed over the top surface of the Si-doped n-type gallium nitride epitaxial lateral overgrowth substrate 100, wherein the n-electrode 115 comprises an Ni layer and an Au layer. The n-electrode 115 is also positioned over the low dislocation density region 104 for current injection into the low dislocation density region 104.
The gallium nitride substrate formed in the above conventional method is so called to as “epitaxial lateral overgrowth gallium nitride substrate”. The sapphire substrate 101 has a high surface dislocation density. The gallium nitride layer 102 grown over the sapphire substrate 101 also has a high dislocation density. The gallium nitride layer 106 has a high dislocation density region 116 positioned over the window regions of the stripe-shaped silicon dioxide mask 103 and a low dislocation density region 104 positioned over the stripe-shaped silicon dioxide mask 103. The high dislocation density region 116 of the gallium nitride layer 106 was grown over the gallium nitride layer 102 having the high dislocation density, for which reason the high dislocation density region 116 of the gallium nitride layer 106 also has a high dislocation density. The stripe-shaped silicon dioxide mask 103 is made of silicon dioxide free of dislocation, for which reason the low dislocation density region 104 grown over the stripe-shaped silicon dioxide mask 103 has a low dislocation density. The high dislocation density region 116 has a high dislocation density of 1×1012 m−2. The stripe-shaped silicon dioxide mask 103 cuts the propagation of the dislocation from the sapphire substrate 101. The low dislocation density region 104 has a low dislocation density of 1×1011 m−2. The low dislocation density region 104 is formed by the epitaxial lateral overgrowth of gallium nitride from the window regions of the stripe-shaped silicon dioxide mask 103. At the center of the low dislocation density region 104, epitaxial lateral overgrowths of gallium nitride in various lateral directions from the window regions of the stripe-shaped silicon dioxide mask 103 come together, whereby new dislocations are formed, for which reason the center region of the low dislocation density region 104 has a high dislocation density. In FIG. 1, the high dislocation density region is represented by dot marks. The p-electrode 105 is formed over the low dislocation density region 104 except for its center region, so that a current is injected from the p-electrode 105 into the low dislocation density region of the active layer. If the current is injected into the high dislocation density region, then the deterioration in performance of the device is likely to appear. However, if the current is injected into the low dislocation density region, then the deterioration in performance of the device is unlikely to appear, resulting in a long life-time of the device.
A second conventional method of forming a low dislocation density region was reported by Nakamura et al. in Applied Physics vol. 68-7, pp. 793-796. A GaN layer is formed on a sapphire substrate. The GaN layer is selectively removed by a dry etching process to form stripe-shaped GaN layers. Further, a GaN layer is formed over the stripe-shaped GaN layers and over the sapphire substrate. The GaN layer is epitaxially grown in vertical direction from the top surfaces of the stripe-shaped GaN layers and also epitaxially grown by an epitaxial lateral overgrowth in lateral direction from the top surfaces of the stripe-shaped GaN layers toward the top surface of the sapphire substrate. This growth will hereinafter referred to as a “mask-less epitaxial lateral overgrowth”. The GaN layer has a high dislocation density region over the stripe-shaped GaN layer and a low dislocation density region over the sapphire substrate. Namely, the low dislocation density region is grown by the epitaxial lateral overgrowth from the stripe-shaped GaN layers to the region over the uncovered top surface of the sapphire substrate. The stripe-shaped GaN layer has a high dislocation density. The dislocation of the stripe-shaped GaN layer is propagated to the high dislocation density region grown in the vertical direction over the stripe-shaped GaN layers. The dislocation of the stripe-shaped GaN layer is propagated to the low dislocation density region grown by the epitaxial lateral overgrowth from the stripe-shaped GaN layers to the region over the uncovered top surface of the sapphire substrate. A current is injected into the low dislocation density region to obtain a long life-time.
The present inventors reported in Jpn. J. Appl. Phys. Vol. 36 (1997), pp. L899-902 and in NEC Research and Development vol. 41 (2000) No. 1 pp. 74-85 that a low dislocation density region is formed in an entire region of the active layer or an entire region of the substrate by a facet-initiated epitaxial lateral overgrowth method using the stripe-shaped silicon dioxide masks. In accordance with the facet-initiated epitaxial lateral overgrowth, the stripe-shaped silicon dioxide masks are formed over the gallium nitride layer over the sapphire substrate to carry out a hydride vapor phase epitaxial growth, wherein the through dislocations are curved, whereby the high dislocation density region as the epitaxial lateral overgrowth is not formed. Thus, the dislocation density is suppressed low over the entire of the substrate.
As the technique for forming the low dislocation density GaN region has been progressed, then the life-time of the blue color nitride based semiconductor laser diode, which emits a laser beam of a wavelength in the range of about 400-500 nanometers, has been greatly improved.
The technique for doping silicon into the active layer has been used for improving the laser device performances such as the threshold current density. In Japanese laid-open patent publication No. 10-12969, it is disclosed that silicon impurity is doped into the active layer at an impurity concentration n the range of 1×1019 m−3 to 1×1021 cm−3 for improvement in the laser threshold value.
In Applied Physics Letter 73 (1998), pp. 496-498 and Proc. of the 2nd Int. Sym. on Blue Laser and Light Emitting Diodes (1998), p. 381, it is disclosed that the active layer is doped with silicon to reduce the threshold value. The mechanism of the threshold value reduction due to the silicon doping process to the active layer might be associated with a piezo electric field shielding effect due to the impurity doping and the improvement in the planarity of the quantum well structure. It has been known that the improvement of the laser device performance is obtainable by the silicon doping. It is the common technical sense that the advanced nitride based semiconductor laser diode has the active layer which is doped with silicon in the above technical viewpoint.
In view of the actual practical use of the laser diode, the reliability of the conventional nitride based semiconductor laser diode is still insufficient. If the nitride based semiconductor laser diode having a wavelength band of about 400-500 nanometers is used for emitting a laser beam for an optical disk such as a digital video disk, a long life-time of not less than 5000 hours at 30 mW and at 70° C. is necessary in consideration of the wiring operation. As reported by Nakamura et al. in JSAP International No. 1, pp. 5-17 (2000), the life-time of the conventional nitride based semiconductor laser diodes is only 500 hours at 30 mW and at 60° C.
In the above circumstances, it had been required to develop a novel nitride based semiconductor photo-luminescent device free from the above problem.